Problem in using the Graph object in different function

I am creating a graph using boost library, I am making the network using create_Network function()
i am adding the vertices to this graph by creating the object g of class Graph.
I am trying to use this object g in the another function get_neighbours()
but it is not giving any result.Should I make it global or should use constructor so that values stored in the graph(vertices) object(g), can be used in any function.